In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required.
The requirement of small features (and close spacing between adjacent features) requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, X-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the photomask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through the photomask causes a chemical transformation in the exposed areas of the coating thereby making the image area either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. Attempts made to improve lithography include advances made in developing new photoresists. For example, so-called deep UV (ultra-violet) photoresists take advantage of improved resolution associated with the relatively short wavelength of light used to expose the deep UV photoresists prior to development. However, some deep UV photoresists are made of acid catalyzed or chemically amplified photoresist materials. This is a concern because problems occur in many instances when using an acid catalyzed photoresist material over a nitride layer.
In particular, acid catalyzed photoresist materials are deleteriously affected or poisoned by nitride layers. Although not completely understood, it is believed that nitrogen atoms from the nitride layer poison a thin portion of the acid catalyzed photoresist material adjacent the nitride layer. More particularly, it is believed that nitrogen atoms at the interface of the nitride layer and the acid catalyzed photoresist act as a Lewis base neutralizing the photogenerated acid preventing chemical change (acid catalysis) within the photoresist following exposure to actinic radiation. Thus, the acid catalyzed photoresist is contaminated as a thin desensitized layer is formed within the photoresist (adjacent the nitride layer) that prevents or deforms subsequent pattern formation of the photoresist.
Referring to FIG. 1, the profile of a poorly developed acid catalyzed photoresist 12 is illustrated. The poorly developed photoresist 12 is formed over a silicon nitride layer 10. This particular poorly developed photoresist 12 exhibits footing, wherein a portion 14 of the developed photoresist is deformed, presumably due to desensitization or acid quenching from the top surface of the silicon nitride layer 10. It is noted that even poorly developed photoresist 12 is difficult to obtain on the top surface of the silicon nitride layer 10, as in many instances development is impossible.
Procedures that increase resolution, improved critical dimension control, and provide small conductive features are desired. These procedures include those that enable the use of high resolution acid catalyzed photoresist materials.